Method of injection molding foamable plastic with minimized wastage

ABSTRACT

THIS DISCLOSURE IS DIRECTED TO A METHOD FOR INJECTION MOLDING. PREMATURE FOAMING AND SETTING OF FOAMABLE MOLDING MATERIALS IS PREVENTED BY ACCOMPLISHING THE INJECTION OF THE FOAMABLE MOLDING MATERIAL INTO A MOLD CAVITY HAVING A GASEOUS SUBSTANCE THEREIN AT A PRESSURE IN EXCESS OF THAT REQUIRED TO PREVENT FOAMING OF THE MOLDING MATERIAL. PLANNED WASTAGE OF MOLDING MATERIAL IS MINIMIZED BY PROVIDING PISTIONS IN THE RUNNERS WHICH CARRY THE MOLDING MATERIALS TO THE MOLD, WHICH PISTONS FORCE EXCESS MOLDING MATERIAL AWAY FROM THE MOLD SO AS TO RECOMPRESS AND RECOVER THE MATERIAL, THEREBY TO BE USED IN THE NEXT SUBSEQUENT MOLDING CYCLE.

NOV. 20, 1973 A, SPAAK ETAL 3,773,873

METHOD 0F INJECTION MOLDING FOAMABLE PLASTIC WITH MINIMIZED WASTAGETTURNEYS 3,773,873 METHOD oF INJECTION MOLDING FOAMABLE PLASTTC WITHNOV. 20, 1973 A, SPAAK ETAL MINIMIZED WASTAGE 3 Sheets-Sheet 3 FiledJune 22, 1970 53 oooov oooo United States Patent O Int. Cl. B29d 27/00U.S. Cl. 264-37 8 Claims ABSTRACT F THE DISCLOSURE This disclosure isdirected to a method for injection molding. Premature foaming andsetting of foamable molding materials is prevented by accomplishing theinjection of the foamable molding material into a mold cavity having agaseous substance therein at a pressure in excess of that required toprevent foaming of the molding material. Planned wastage of moldingmaterial is minimized by providing pistons in the runners which carrythe molding materials to the mold, which pistons force excess moldingmaterial away from the mold so as to recompress and recover thematerial, thereby to be used inthe next subsequent molding cycle.

RELATED APPLICATIONS This application is a continuation-in-part of ourcopending application Ser. No. 811,271 for Method of Injection MoldingFoamable Plastic which was filed on Feb. 19, 1969, which, in turn, is acontinuation-in-part of our application Ser. No. 679,3 87 for Method andApparatus for Molding Articles, filed on Oct. 3l, 1967, and now bothabandoned, which are both assigned to the same assignee as the assigneeof the invention of this application.y

BACKGROUND OF THE INVENTION tively high strength while being ofrelatively low weight.

The injection method which has been used'inzproducing the high densityshell about the lower density, foam core has included the steps of:providing in a .mold cavity a charge of a pressurized gas-expandablemolding material .containing a blowing agent; maintaining injectionpressure against the charge to preclude the blowing agent from expandingprior to the formation of the high density shell; cooling the mold toform a hardened shell of molding material adjacent the mold surface;reducing the pressure on the charge to allow the expansion of theblowing agent in the unhardened portion of the charge so as to cause theproduction of a foam structure within the shell and expulsion of excessmaterial from the mold cavity; and cooling the formed article to finallyset the foarned molding material. v f

Two difficulties have been encountered in attempting to practice theabove-described method. First, the unfoamed charge of molding materialhasv exhibited the tendency to foam and set during injection orotherwise prematurely, resulting inv either an incomplete product or ina product not having the desired hardened shell around a foam center.Secondly, the process has resulted in exlCC cessive planned wastage ofmolding material which, althrough subject to recovery and ultimate use,requires further treatment, thus increasing the cost of themanufacturing operation.

1 Considering first the problem of premature setting and foaming, someknown injection molding methods and apparatus ordinarily injectcompressed molding material containing a blowing agent into a moldwherein provision is made for rapid cooling of the outer portion of theinjected material prior to expansion so as to achieve the desiredhardened shell around a foam center. The mold cavity, initially being ata pressure substantially below the vapor pressure of the blowing agentin the molding material being injected, provides a reduced-pressureenvironment into which the molding material being injected may foambecause of the reduction in pressure on the blowing agent to a pointbelow that required to prevent foaming. Thus, immediately upon enteringthe mold cavity, the mass of molding material starts expanding and,since the mold is ordinarily at a substantially lower temperature thanthe molding material, shortly after the commencement of expansion, theexpanded material starts to set. Thus, the unfavorable combination ofpressure and temperature conditions in the mold cavity tend to causepremature setting of the molding material which often occurs in the areanear the injection nozzle causing clogging, variations from desiredproduct consistency, and wastage.

Additionally, most known methods and apparatus for the injection moldingof foaming molding materials have a relatively high degree of plannedwastage inherent in their practice and use because of the amounts ofmaterial left in runner and sprue passages. Attempts to reduce thiswastage have been made, including that o f allowing excess material toflow back through the system during expansion 4to become part of thenext batch of material to be injected. The attempts have been met withdifficulty, however, particularly with the problem of the excessmaterial setting during return ow, thus causing clogging of theinjection nozzle.

Accordingly, it is the object of the present invention to provide amethod of and apparatus for injection molding of foamable thermoplasticmaterial wherein premature setting is precluded and wastage of materialis minimized.

SUMMARY O-F THE INVENTION A method of injection molding according to theteaching of the present invention may include the steps of introducing agas into a mold cavity at a pressure sufficient to prevent foaming of afoamable molding material and injecting the foamable material into thecavity having pressurized gas therein. Additional aspects of the presentinvention may include; controllably exhausting gas from the mold cavityduring injection; controlling the temperature of the mold before,during, and after the injection of--molding material; and forcing excessfo'amable material away vfrom the mold cavity after, and selectivelyalso rbefore allowing the material to foam so 'as to recompress theexcess molding material for use as part of a charge for the nextsubsequent molding cycle. l

An apparatus in accordance with the teaching of the present inventionmay include a mold having a cavity therein, means for injecting foamablemolding material into the mold cavity, and means for introducing apressurized gaseous substanceinto the mold cavity prior to injecting soas to prevent foaming of the molding material during injection.

BRI-EF DESCRIPTION 0F THE-DRAWINGS tages may be had from a considerationof the following detailed description, particularly when read in thelight of the attached drawings wherein:

FIG. 1 is an elevational view, partly in section of an apparatusaccording to the present invention; w

FIG. 2 is an elevational view, partly in section and similar to FIG. l,but showing only a portion of the mold apparatus in a stage of thepresent method subsequent to the stage shown in FIG. 1;

FIG. 3 is a view similar to that of FIG. 2, but showing a stage of thepresent method subsequent to the stage shown in FI'G. 2; and

FIG. 4 is a view similar to that of FIGS. 2 and 3, but showing a stageof the present method subsequent to the stage shown in FIG. 3.

DETAILED DESCRIPTION Referring now to FIG. 1, there is shown a novelinjection molding apparatus, designated generally by the refer-Y encenumeral 10, for practicing an injection molding method according to theteaching of the present invention'.

Apparatus comprises a molding section, designated generally by thereference numeral 12, and a melting and injection section designatedgenerally by the reference numeral 14. Molding section 12 includes aframe having a front plate and a back plate 21 connected by four railmembers 23, 24, and 26. A ram 28 passes freely through back plate 21 toreciprocably drive a mold plate 29 which is slidably mounted on railmembers 23 through 26. Mold plate 29 serves as a mounting member for aretractable mold section 30 which cooperates with a stationary moldsection 32 to define a mold cavity 33, as will be discussed hereinafterin greater detail.

Positioned between stationary mold section 32 and front plate 20 is arunner plate 35. Runner plate 35 is in surface-to-surface contact on oneside with the base of stationary mold section 32, and is insurface-to-surface contact on its other side with front plate 20. Runnerplate 35 is provided with a runner passage 38 extending verticallytherethrough from top through bottom, which passage reciprocablyreceives runner pistons 39 and -40 through its upper and lower endsrespectively. Pistons 39 and 40 are operated by liuid motors 42 and 43respectively, as will be discussed in greater detail. A pair oflongitudinally extending sprue passages 44 communicate runner passage 38with mold cavity 33. Each of the sprue passages 44 is spaced outwardlyof the center of passage 38, one each located at pointssubstantiallymidway between the center of passage 38 and its upper and lowerextremities.

A passage 41 is provided in stationary mold section 32 for communicatingmold cavity 33 with a line source of pressurized nitrogen, as will bediscussed further. A relief valve 45 is provided in the nitrogen sourceline to preclude buildup of nitrogen pressure in cavity 33 beyondpredetermined limits. These limits are in the range of being greaterthan the vapor pressure of the blowing agent being utilized, but lessthan the injection pressure of the molding material. Also formed in theretractable and stationary mold sections are a plurality of passages 51.Passages 51 define a flow path for mold cooling fluid, the fluid beingprovided from suitable sources (not shown).

Disposed in the mold sections 30, 32 between passages 51 are a pluralityof heater elements 50 for selectively heating the mold sections. In theapparatus disclosed, resistance heating elements are shown, but it is tobe recog# nized that heating can also be accomplished by other meanssuch as the circulation of hot. tiuid through suitable passages, or bythe use of radiant heaters.

lFront plate 20 is provided with a central bore 46 which extends axiallytherethrough and communicates with a coaxial bore'47 in runner plate 35.A pair of coaxial counterbores 48 and 49 are provided in the outersurface 50 of front plate 20 to accommodate the insertion of aninjection nozzle 52 of a charge barrel 53, as will be discussed ingreater detail. COLlDGrbOIS 48 iS 0f rela- 4 tively smaller diameter andspaced inwardly of counterbore 49.

Charge barrel 53 comprises a generally cylindrical barrel having areduced diameter portion at one end deiining the injection nozzle 52.Nozzle 52 has a passage 54 extending longitudinally axiallytherethrough, which passage cooperates with bores 46 and 47 to define aninjection passage for communicating runner passage 38 with a melting andinjection chamber 56 within charge barrel 53. Barrel 53 is provided withsuitable electric heating bands 55 which serve to melt the charge ofmolding material. A quick opening valve is provided in nozzle 52 toselectively interrupt and allow the ow of molten molding materialthrough passage 54.

Chamber 56 of charge barrel 53 is generally cylindrical having its otherend open to accommodate the insertion of a rotatable, reciprocable screwdrive 58. As will be discussed further in greater detail, screw drive 58charges barrel 53 with molding material which is introduced to chamber56 from a suitable hopper 59 through a passage 61 in the barrel wall.

Screw drive 58 is provided with two distinct sources of motive power.The first, a rotational drive designated generally by the referencenumeral 62, comprises a motor 63 driving a pinion 64 which is in meshingengagement with a gear 65 formed on the shaft of screw drive 58.Actuation of motor 62 by suitable control apparatus (not shown) causesthe rotation of screw drive 58 in either the clockwise orcounterclockwise direction as desired.

The second source of motive power for screw drive 58 is a reciprocatingdrive designated generally by the reference numeral 66. Reciprocatingdrive 66 comprises generally a fluid cylinder 68 having a piston 69mounted for reciprocation and rotation therein, which plston separatesthe interior of cylinder 68 into two chambers 70 and 71. Piston 69 isrigidly secured on one side to a rod 72 which is also connected to screwdrive 58 adjacent gear 64. The opposed ends of cylinder 68 are incommunication with a hydraulic control system including a pump 73,reservoir 74 and control valve 75 so that pressurized Huid may beselectively introduced or withdrawn from chambers and 71 to suitablyreciprocate piston 69. In that screw drive 58 is rigidly secured topiston 69 by rod 72, it is evident that as piston 69 is displaced backand forth within cylinder 68, screw drive 59 is carried therewith andcaused to act as a piston within chamber 56.

In operating the above-described apparatus according to the teaching ofthe present invention, ram 28 is advanced to the position shown in FIG.1, thus positioning retractable mold 30 adjacent stationary mold 32 todeine mold cavity 33. It is to be recognized that with the molds 30, 32in the position shown in FIG. 1, their engagement is such as to rendermold cavity 33 fluid tight.

If the nature of the material to be molded warrants heating to obtain ahigh degree of surface gloss, heater elements 50, optionally provided,are then actuated in both the retractable and stationary mold sectionsto a tempera-4 l ture which obviates the possibility of prematurehardening of the molding material upon injection. It has been found thatalthough preheating of the molds is optional in the practice of thepresent invention, preheating to a temperature in the range of 200 F. to400 F. is preferable for most applications; however, other temperaturesmay be found to be more appropriate depending upon the material beingmolded. Generally the higher mold temperatures are helpful in obtaininga good surface finish. Concurrently with the heating of mold sections30, 32, a charge of a gaseous substance such as pressurized nitrogen isintroduced to mold cavity 33 through passage 41. The pressure of thenitrogen should be higher by some degree thanl that pressure below whichthe molding material will commence foaming so as to preclude foaming ofthe molding material during injection, but lower than the pressure ofinjection so as not to interfere with the injection process. It is to benoted, also, that while the present iriscrew drive 59, thus forcingmolding material into chamber 56 from hopper 69 and toward the nozzle 52of barrel 53. In this regard, it is to be recognized that the method andapparatus according to the teaching of the present invention may be usedfor the injection molding of virtually any molding material, e.g.,foamable thermoplastic resins. Thus, the present method of -foam moldingcan be practiced using available materials, such as high and low densitypolyethylene, rigid and exible polyvinylchloride, general purposepolystyrene, impact grades of polystyrene,

,acrylonitrile-butadiene-styrene (ABS) polymer, polyphenylene oxide,polycarbonate, polypropylene, ethylenevinyl acetate copolymer and nylon.Any of the above materials may be blended with a blowing agent in thepractice of the present invention Hence, for example, the present methodof foam molding has been practiced using commercially high densitypolyethylene copolymer (containing about 1% of butene-l), dry blendedwith up to two percent (2%) by weight of azodicarbonamide(NH3-CO-NH-CO-NH2) as a blowing agent. Additionally, other blowingagents Iwhich may be utilized are the low boiling alcohols such asmethanol and propanol; the low boiling ethers such as dimethyl ether andchlorofluorovalkanes such as l,2-dichloroetetrauoroethane. Each of theabove listed resins has been utilized in the practice of the inventionand, in this regard, no such resin has been found which cannot be usedin the present process, it

being only necessary to change the type and/or percentage of blowingagent which is within the. skill of those who are skilled in the art,certain aspects of the present invention have utility in the injectionmolding of nonfoaming, as well as foaming molding materials.

The advance of molding material within barrel 53 by rotation of screwdrive 58 causes the screw drive 58 to be displaced rightwardly (as seenin FIG. 1) as chamber 56 is filled. Thereafter, when the moldingmaterial is thoroughly melted and mixed with the blowing agent,hydraulic control system 66 is actuated to withdraw the iiuidfromkcyclinder chamber 70 and inject fluid into cylinder chamber 71thereby tending to displace piston 69 and therewith screw drive 58 tothe left from the position shown in FIG. 1, to the position shown inFIG. 2. Displacement of screw drive 58 to the left causes an increase ofpressure in the charge in chamber 56 in anticipation of the actualinjection.

When the pressure in chamber 56 is suitably high, val-ve 60 is openedand the pressurized molding material is injected through passages 54,46, and 47 to runner passage 38 and thereafter through sprues 44 intomold cavity 33. The pressurized nitrogen in cavity 33 precludespremature foaming of the molding material but does not interfere withinjection since as injection progresses, ex-

' cess nitrogen is controlledly exhausted from the cavity through reliefvalve 45 which is preset at some pressure greater than the vaporpressure of the blowing agent and less than the injection pressure ofthe molding material. It is to be noted that during the injection ofmolding material, runner pistons 39 and 40 are maintained in theretracted position so as to not interfere with the ow of materialthrough runner passages 38 and sprues v44.

Upon completion of the.. injection, the apparatus is oriented as shownin FIG. 2, mold cavity 33 is filled with molding material and pressureis maintained upon the molding material by the compressive force ofscrew drive 58 while a s olid outer layer or skin ofv material is beingformed within the mold. The pressureexerted by screw 58 is maintained topreclude foaming of the molding material prior to the complete formationof the hard outer layer of material. The thickness of the outer layer iscontrollable by suitably regulating the temperature of mold sections 30,32 by an appropriate balance of hot and cold fluids flowing throughpassages 50 and 51 respectively, and by selectively controlling theamount of time during which injection pressure is maintained on theinjected material.

Once a solid skin of plastic material has been formed as desired, screwdrive 58 is retracted by reciprocating control system 66 (FIG. 3).Retraction of screw 58 relieves the pressure on the molding material,thus allowing the unfoamed material contained within the hardened shellto expand and generate a foamed central portion. As the unfoamed,unhardened material expands, an excess is generated which flows back outof sprues 44, into runner passage 38 and ultimately back into chargebarrel 53.

Once sufficient expansion has been permitted, fluid motors 43 and 42 areactuated to advance pistons 39 and 40 into runner passage 38 (FIG. 4) soas to cover sprues 44, recompress the excess material contained thereinand force the excess molding material back through the injectionpassage, i.e. runner passage 38, and passages 46, 47 and 54, towardcharging barrel 53. The temperature in mold sections 30 and 32 is thenlowered by reducing the heating rate of heaters 50 while maintaining aflow of cold iluid in passages 51. With the mold temperature thuslowered, the molded article hardens, after which ram 28 is actuated toretract mold section 30 so as to permit removal of the product 80, seeFIG. 4. When the product is removed, pistons 39 and 40 are retracted,mold section 30 is advanced to molding position and the entire cycle isrepeated.

The apparatus of the present invention may be operated to practice analternative method according to the teachings of the present invention.

Considering this alternative method, and referring again to FIG. 1,start-up of the cycle is commenced by lling chamber 56 of charge barrel53 with foamable molding material and pressurizing the foamable moldingmaterial in the same manner as discussed above. Also, ram 28 is advancedto position retractable mold 30 adjacent stationary mold 32 to definefluid tight mold cavity Mold sections 30, 32 are then preheated byheater elements 50, and a charge of pressurized gaseous substance, suchas pressurized nitrogen, is introduced to the mold cavity 33 throughpassage 41. As discussed above, the pressure of the gas should be higherby some degree than that pressure below which themolding material willcommence foaming so as to preclude foaming of the molding materialduring injection. Additionally, during startup, pistons 39 and 40 shouldbe maintained retracted so that the pressurized gas lls not only moldcavity 33 and sprues 44 but also runner passages 38, central bore 46 offront plate 20, and injection nozzle 52 back to quick opening Value 60.In subsequent cycles, however, as will be discussed below, the pistons39 and 40 are maintained advanced prior to injection and thus, thepressurized gas lls only mold cavity 33 and sprues 44.

With the barrel chamber 56 and mold cavity 33 charged with moldingmaterial and pressurized gas respectively, valve 60 is opened and thepressurized molding material is injected through passages 54, 46 and 47,into runner passage 38, and thereafter through sprues 44 into moldcavity 33. During this injection, and in the same manner discussedabove, the pressurized gas is relieved from mold cavity 33 throughrelief valve 45 in such a manner as to maintain the molding materialunder pressure which is suiciently great to preclude premature foamingand suficiently small to not interfere with injection.

Upon completion of the injection of molding material into mold cavity 33(FIG. 2), the temperature of the mold sections 30, 32 is suitablyregulated to cause the formation of a hard outer layer of materialadjacent the surface of the mold sections. Concurrently with theformation of the hard outer layer of material, runner pistons 39 and 40are advanced in runner passage 38 to force the pressurized unusedmolding material back toward charge barrel 53 (i.e. runner pistons 39and 40 advance from their positions as shown in FIG. 3 to their positionas shown in FIG. 4). The advance of pistons 39 and 40 to cause the owback of unused molding material generates a pressure against screw drive58 'which is sufficient to displace screw drive 58 away from nozzle 52at a rate which permits the receipt of the unused material to passage54.

When pistons 39 and 40 are fully advanced, valve 60 is closed.Thereafter, when the outer layer of material is formed to the desiredthickness, pistons 39 and 40 are retracted within runner passage 38. Theretraction of pistons 39 and 40 uncovers sprues 44 and also creates avoid in passage 38 into which excess molding material may flow duringfoaming. Specifically, by uncovering sprues 44, the pressure on themolding material in mold cavity 33 is relieved and foaming thuscommences. As foaming progresses, an excess volume of material isgenerated which flows out of sprues 44 an-d into the void in runnerpassage 38 generated by the retraction of pistons 39 and 40. p

If the volume of the void in runner passage 38 is sufficient toaccommodate allthe excess foamed material generated in the manufactureof a particular article, then upon the filling of the void in runnerpassage 38, pistons 39 and 40 are advanced to once again cover sprues44, valve 60 is opened, and the foamed material is recompressed andforced back toward chamber 56 of charging barrel S3. It is to berecognized, however, that the volume of excess foamed material generatedin the manufacture of a particular article may be greater than thevolume of the void of runner passage 38. In this situation, the void maybe filled in the manner discussed above, pistons 39 and 40 maythereafter be advanced to recompress the foamed molding material andforce the foamed molding material back into chamber 56 and thereafter,valve 60 may again be closed and pistons 39 and 40 once again retractedto reestablish a void for receiving additional excess material. Thisprocess may be repeated as often as may be necessary to provide for theremoval of all the excess material generated without suffering thewastage iof any of this material.

Once sufficient expansion has been permitted and the excess materialcarried away for use in the next subsequent cycle, pistons 39 and 40 aremaintained in their advanced positions (FIG. 4) so as to maintain sprues44 covered, and the temperature in mold sections 30 and 32 is suitablycontrolled by the cooperation of heaters 50 and cooling fluid inpositions 51 to cause hardening of the molded article. Thereafter, moldsection 30 is retracted and the product 80 is removed (FIG. 4).

After the product 80 is removed, retractable mold 30 is repositionedadjacent stationary mold 32 to once again define mold cavity 33. Apressurized gaseous substance is introduced into mold cavity 33 to fillmold cavity 33 and sprues 44. Thereafter, with a fresh charge offoamable molding material, which charge includes that material which wasrecovered as excess and unused material from the prior cycle, screwdrive 58 is advanced and pistons 39 and 40 are retracted within runnerpassage 38 so as to uncover sprues 44 and to allow injection of the newcharge lof molding material into mold cavity 33 as discussed above.Thereafter, the full molding cycle may be repeated as often as isdesired to form as many articles 80 as are necessary.

It can be seen, therefore, that the above-described alternative methodof utilizing the apparatus of FIGS. 1-4 provides for the discharge ofexcess foamable material into a void created by the reciprocation ofpistons 39 and 40'. Whereas', the first method described above providesfor the discharge of excess foamed material into a space created by theretraction of screw drive 58 within chamber 56.

It is to be recognized that although the above-described methods eachinclude the steps of providing a pressurized gaseous substance withinmold cavity 33 prior to the injection of molding material, there arearticles which may be formed using aspects of the methods and apparatusof the present invention, which articles are configured in such amanner, or which articles are sufiiciently small, to render the elect ofthe problem of premature foaming and setting negligible. In thesesituations, the present invention may be practiced without providing thepressurized gaseous substance in the mold cavity while still obtainingthe advantages available through the positive recovery of the unused andexcess material.

It should be understood, of course, that many modifications andvariations of the present invention are possible in the light of theabove teachings without departing from the spirit and scope of theinvention. Accordingly, the present invention contemplates thesemodifications and variations as falling clearly within the scope of thepresent inventive concept.

We claim:

1. The method of injection molding a foamable thermoplastic moldingmaterial including the steps of:

melting a charge of foamable thermoplastic molding material so as toprepare the charge for injection into a mold having a mold cavity;

injecting said foamable molding material through passages into said moldcavity at a pressure sufficiently high to prevent foaming;

allowing said molding material under pressure to set in said mold cavityso as to take on the shape of said mold cavity and form a solid skinaround the inner surface of said mold cavity;

relieving the pressure from said foamable molding material so as toallow the foaming thereof within said solid skin and the expulsion ofexcess material out of said mold cavity into said passages; and

applying external force to said excess molding material in said passagesto displace said excess molding materialV away from said mold cavity andback through said passages and to recompress said molding material foruse as a part of a charge for a next subsequent molding cycle.

2. The method of claim 1 wherein said step of applying external force toexcess molding material in said passages comprises the step of:

advancing piston means into said passages to force excessmoldingmaterial away from said mold cavity. 3. The method of claim 1wherein said foamable molding material is a polypropylene.

4. The method of claim 1 wherein said foamable molding material is apoly propylene.

5. The method of claim 1 wherein said foamable molding material is apolystyrene.

6. The method of claim 1 wherein said foamable molding material is anylon.

7. The method of injection molding a foamable thermo plastic materialincluding the steps of:

preparing a charge of foamable thermoplastic molding material forinjection into a mold having a mold cavity; l

injecting said foamable molding material through passages into saidmoldcavity at a pressure sufficiently high toprevent foaming;

advancing piston means to apply force to unused foamable moldingmaterial remaining in said passages after the filling of said moldcavity to displace said unused foamable molding material away from saidmold cavity and back through said passages;

forming a void in said passages by retracting said piston means fromsaid passages;

allowing said molding material under pressure to set in said mold cavityso as to take on the shape of said mold cavity and form a solid skinaround the inner surface of said mold cavity;

relieving the pressure from said foamable molding material in said moldcavity to allow the foaming thereof within said solid skin and theexpulsion of excess molding material out of said mold cavity and intosaid void; and

advancing said piston means to apply force to said excess moldingmaterial in said void to recompress said molding material for use as apart of a charge, for a next subsequent molding cycle.

8. A method of injection molding a foamable thermoplastic moldingmaterial including the steps of:

melting a charge of foamable thermoplastic molding material selectedfrom the group consisting of polyethylene, polystyrene, polypropyleneand nylon s as to prepare the charge for injection into a mold having amold cavity;

injecting said foamable molding material through passages into said moldcavity at a pressure sufciently high to prevent foaming;

allowing said molding material under pressure to set in said mold cavityso as to take on the shape of said mold cavity and form a solid skinaround the inner face of said mold cavity;

relieving the pressure from said foamable molding material so as toallow the foaming thereof within said solid skin and the expulsion ofexcess material out of said mold cavity into said passages; and

applying external force to said excess material in said passages todisplace said excess molding material away from said mold cavity andback through said 10 passages and to recompress said molding materialfor use as a part of a charge for a next subsequent molding cycle.

References Cited UNITED STATES PATENTS 3,694,529 9/ 1972 Josephsen etal. 264-55 X 2,613,395 10/1952 Massler 264-329 X 2,698,460 1/1955 Amo264--329 X 2,698,464 1/1955 Wilson 264--329 X 2,829,117 4/1958 Lindemann264-51 X 3,218,375 11/1965 Hardwick 264-51 X 3,436,446 4/ 1969 Angell264-51 3,268,635 8/1966 Kraus 264-55 X 3,211,605 10/196'5 Spaak 264-51 X3,389,198 6/1968 Taber 264-54 3,468,990 9'/1969 Odell 264-51 X 3,268,6368/1966 Angell 264-51 3,162,703 12/1964 Eyles 264-51 3,437,722 4/ 1969Cronin 264-51 X 3,249,660 5/1966 King 264-51 FOREIGN PATENTS 723,0444/1969 Belgium. 1,590,373 5/ 1970 France.

OTHER REFERENCES Derwent Belgian Patents Report, Derwent Publications,

London, No. 18/69, Plastics and Metal Finishing, p. 1, July 7, 1969.

PHILIP E. ANDERSON, Primary Examiner U.s. c1. X'.R.

264-48, 53, '54, 328, 329, DIG. 5, DIG. 14, DIG. 17, DIG. 69, DIG. 83

gg@ UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No.3,773,873 Dated November 20. 1973 1nventor(s) Albert. Speak and.Clifford L. Weir It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Coluz'rzn 5, lino 9, "59" should read 58 lino 10, "69" .Should road 59Ine 23, after' "invention", insert line 38, after "art" insert Further',will be recognized. by those ollzjlled in the art,

Co] umn 6, Iinf.: 58, "value" nhouI/ row? valve.

Colman 7, limo B, "yooitions" :would reed position Column E, line 5,

il, after "plastic" insertmoldng;

line

Signed and sealed this 16th day of April 197k..

(SEAL) Attest:

EDWARD E.ELETGHER,JR. t -V C. MARSHALL DANN M t Attestng OfficerCommissioner of Patents

